There have been known wavelength tunable lasers and monolithic integrated lasers with an optical switch as semiconductor lasers integrated with modulators.
FIGS. 1 and 4 show typical structures of wavelength tunable lasers.
A wavelength tunable laser basically includes a diffractive grating on the same layer as the one forming an optical waveguide, and changes the wavelength of light reflected from the diffractive grating by varying the refractive index thereof to thereby tune the lasing wavelength.
The device shown in FIG. 1 is a wavelength tunable, multiple-electrode-distributed Bragg reflector (DBR) laser diode wherein an optical waveguide layer 132 is provided between two semiconductor layers 131 and 134 of different conductivity of p or n, an active layer 133 is provided in a region on an end of the optical waveguide layer 132, and a diffractive grating 135 is provided on the other end thereof. The electrodes of the device are divided in three regions. The first electrode 136 is provided on the region of the active layer 133 in order to supply electric current required for lasing. The second electrode 137 is formed on a region where no active layer 133 nor diffractive grating 135 is provided. The third electrode 138 is provided on a region of the diffractive grating 135.
The device is of a pn structure entirely in the vertical direction, and therefore can change the carrier density on the optical waveguide 132 by the electric current passed through the pn structure to eventually change the refractive index. Such change in the refractive index is known as plasma effect.
FIGS. 2 and 3 show examples of changes in the spectral line width and the lasing wavelength as against the electric current I.sub.p +I.sub.b passed through the electrodes 137 and 138 respectively. In these examples, by changing the electric current I.sub.p +I.sub.b by 70 mA, it can vary wavelength by 3.1 nm (or 380 GHz in frequency). The structure and characteristics of the device are taught by S. Murata et al., Electron Lett., Vol. 23, p. 403, 1987.
The device shown in FIG. 4 is a wavelength tunable multiple-electrode distributed feedback (DFB) laser diode wherein an active layer 162 is provided between two semiconductor layers 161 and 163 having different conductivity of p or n, and a diffractive grating 164 is provided along the active layer 162. Electrodes of the device are divided into an electrode 165 for lasing and an electrode 166 for controlling the refractive index in the direction of beams. An anti-reflection coating 167 is formed at an emergent end.
The device is described in detail by M. Kuznetsov, in IEEE. QE-24, No. 9. p. 1837, Sept. 1988.
Similarly to the wavelength tunable multiple electrode distributed Bragg reflector (DBR) laser diode, the device changes the carrier density by the electric currents I.sub.1, I.sub.2 of electrodes 165, 166 to thereby vary the refractive index. The lasing wavelength is controlled with the electric current I.sub.2 of the electrode 166 for controlling the refractive index while maintaining the total injection current I.sub.t =I.sub.1 +I.sub.2 constant.
FIG. 5 shows an example of the spectral line width electrodes 165 and 166. The characteristics thereof are disclosed by Kenji Sato et al. in OFC' 89, Feb. 1989, TUH 3.
There is also known an integrated laser with an optical modulator which does not use the plasma effect caused by injection of electric current. FIGS. 6, 7a and 7b show such an example.
FIG. 6 shows the structure of an integrated laser with an optical switch using a multiple quantum well structure in the modulator thereof. The figure is partially broken away in order to show the interior structure The device can change the intensity of light at a high speed by changing optical characteristics (absorption) of the optical waveguide by applying an electric field on the multiple quantum well structure (MQW). The device is taught by Y. Kuwamura, K. Wakita et al. in OQE 86-169.
FIGS. 7a and 7b show energy band structures of a laser having a pnp transistor structure (npn would be similar to that shown in the figures). Corresponding to the transistor, respective layers are made an emitter, a base and a collector. FIG. 7a shows the state where no electric voltage is applied between the base and the collector while FIG. 7b shows the state applied with voltage.
This device uses a pn junction between the emitter and the base as an active layer, and forms an optical gain layer for lasing by injecting the electric current thereto in forward bias. By varying the bias applied between the base and the collector, the carrier confinement effect of the active layer is varied to change the optical gain. This permits modulation of light intensity.
Those devices are described by Suga, Yamanishi et al. in the Proceedings of 1988 Autum Conference of Applied Physics Association, Article No. 5a-R-6.
However, the wavelength tunable multiple-electrode DBR laser diodes are defective in that if the lasing wavelength changes because of the plasma effect, it not only changes the refractive index but also increases the optical absorption and heat generation by free carriers to change the threshold gains of laser, to lower the optical output or to increase the spectral line width as shown in FIG. 5.
In the case of a wavelength tunable multiple-electrode DFB laser diode, as the optical absorption hardly changes in a resonator as a whole, even if the lasing wavelength is varied, it would not change the optical output or the spectral line width. But when electric current for controlling the refractive index I.sub.2 increases beyond the lasing threshold current, almost all of the injected electric current is consumed for the increment in optical output, and the carrier density ceases to change. This inconveniently limits the tunable width of lasing wavelength.
Further, the conventional integrated laser with optical modulator is complicated in structure, and hence the manufacture process is complex and difficult.